Hybrid OLED having phosphorescent and fluorescent emitters

ABSTRACT

The present invention provides organic light emitting devices having a combined emission from at least two emissive materials, a fluorescent blue emissive material and a phosphorescent emissive material. The device may further comprise additional fluorescent or phosphorescent emissive materials. In preferred embodiments, the invention provides OLEDs having three different emissive materials—a red emissive material, a green emissive material and a blue emissive material. The invention provides a device architecture which is optimized for efficiency and lifetime by using a combination of fluorescent and phosphorescent emitters. Further, in preferred embodiments the device architecture provides a high color-stability of the light emission over a wide range of currents or luminances.

JOINT RESEARCH AGREEMENT

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a joint universitycorporation research agreement: Princeton University, University ofSouthern California, and Universal Display Corporation. The agreementwas in effect on and before the date the claimed invention was made, andthe claimed invention was made as a result of activities undertakenwithin the scope of the agreement.

FIELD OF THE INVENTION

The present invention relates to organic light emitting devices (OLEDs),and more specifically to OLEDS that emit light using a combination offluorescent emitters and phosphorescent emitters. In preferredembodiments the combined emission from the fluorescent andphosphorescent emitters sufficiently span the visible spectrum to give awhite emission.

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be an fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules. In general, a small molecule has a well-definedchemical formula with a single molecular weight, whereas a polymer has achemical formula and a molecular weight that may vary from molecule tomolecule. As used herein, “organic” includes metal complexes ofhydrocarbyl and heteroatom-substituted hydrocarbyl ligands.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

OLED devices are generally (but not always) intended to emit lightthrough at least one of the electrodes, and one or more transparentelectrodes may be useful in an organic opto-electronic devices. Forexample, a transparent electrode material, such as indium tin oxide(ITO), may be used as the bottom electrode. A transparent top electrode,such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which areincorporated by reference in their entireties, may also be used. For adevice intended to emit light only through the bottom electrode, the topelectrode does not need to be transparent, and may be comprised of athick and reflective metal layer having a high electrical conductivity.Similarly, for a device intended to emit light only through the topelectrode, the bottom electrode may be opaque and/or reflective. Wherean electrode does not need to be transparent, using a thicker layer mayprovide better conductivity, and using a reflective electrode mayincrease the amount of light emitted through the other electrode, byreflecting light back towards the transparent electrode. Fullytransparent devices may also be fabricated, where both electrodes aretransparent. Side emitting OLEDs may also be fabricated, and one or bothelectrodes may be opaque or reflective in such devices.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. For example, for a devicehaving two electrodes, the bottom electrode is the electrode closest tothe substrate, and is generally the first electrode fabricated. Thebottom electrode has two surfaces, a bottom surface closest to thesubstrate, and a top surface further away from the substrate. Where afirst layer is described as “disposed over” a second layer, the firstlayer is disposed further away from substrate. There may be other layersbetween the first and second layer, unless it is specified that thefirst layer is “in physical contact with” the second layer. For example,a cathode may be described as “disposed over” an anode, even thoughthere are various organic layers in between.

As used herein, “solution processible” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

As used herein, and as would be generally understood by one skilled inthe art, a first “Highest Occupied Molecular Orbital” (HOMO) or “LowestUnoccupied Molecular Orbital” (LUMO) energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. Since ionization potentials(IP) are measured as a negative energy relative to a vacuum level, ahigher HOMO energy level corresponds to an IP having a smaller absolutevalue (an IP that is less negative). Similarly, a higher LUMO energylevel corresponds to an electron affinity (EA) having a smaller absolutevalue (an EA that is less negative). On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material. A“higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

The quality of white illumination sources can be fully described by asimple set of parameters. The color of the light source is given by itsCIE chromaticity coordinates x and y (1931 2-degree standard observerCIE chromaticity). The CIE coordinates are typically represented on atwo dimensional plot. Monochromatic colors fall on the perimeter of thehorseshoe shaped curve starting with blue in the lower left, runningthrough the colors of the spectrum in a clockwise direction to red inthe lower right. The CIE coordinates of a light source of given energyand spectral shape will fall within the area of the curve. Summing lightat all wavelengths uniformly gives the white or neutral point, found atthe center of the diagram (CIE x,y-coordinates, 0.33, 0.33). Mixinglight from two or more sources gives light whose color is represented bythe intensity weighted average of the CIE coordinates of the independentsources. Thus, mixing light from two or more sources can be used togenerate white light.

When considering the use of these white light sources for illumination,the CIE color rendering index (CRI) may be considered in addition to theCIE coordinates of the source. The CRI gives an indication of how wellthe light source will render colors of objects it illuminates. A perfectmatch of a given source to the standard illuminant gives a CRI of 100.Though a CRI value of at least 70 may be acceptable for certainapplications, a preferred white light source may have a CRI of about 80or higher.

White OLEDs having only phosphorescent emissive materials may be highlyefficient, but their operation stability is currently limited by thelifetime of the blue phosphorescent component. White OLEDs having allfluorescent emissive materials may have good operational stability, buttheir external quantum efficiency is generally about three times lowerthan that of all-phosphorescent white OLEDs. The present inventioncombines phosphorescent and fluorescent technologies in an improveddevice structure to achieve an improved balance of efficiency andlifetime in a white OLED.

SUMMARY OF THE INVENTION

The present invention provides organic light emitting devices having acombined emission from at least two emissive materials. Morespecifically, the present invention provides an organic light emittingdevice comprising a cathode, a first emissive layer comprising afluorescent blue emitting material, a second emissive layer comprising aphosphorescent emitting material, and an anode, wherein when a voltageis applied across the device, light is emitted from the emittingmaterials. In preferred embodiments, the combined emission of theemissive materials sufficiently spans the visible spectrum to give awhite emission. In particularly preferred embodiments, the devicefurther comprises a spacer layer between the first emissive layer andthe second emissive layer.

Another embodiment of the present invention an organic light emittingdevice comprising an emissive region disposed between and electricallyconnected to an anode and a cathode, wherein the emissive regioncomprises a fluorescent blue emitting material, a phosphorescent greenemitting material; and a phosphorescent red emitting material. Thefluorescent blue emitting material is in a separate emissive layer fromthe phosphorescent red emitting material and the phosphorescent greenemitting material, and the combined emission of the emitting materialssufficiently spans the visible spectrum to give a white emission. Inparticularly preferred embodiments, the device further comprises aspacer layer between the emissive layer comprising the fluorescent blueemitting material and the emissive layer comprising the phosphorescentemissive materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device having separate electrontransport, hole transport, and emissive layers, as well as other layers.

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

FIG. 3 shows the device structure for the devices of Examples 1 and 2.

FIG. 4 shows the device structure for the devices of Example 3.

FIG. 5 shows the device structure for the devices of Examples 4-9.

FIG. 6 shows external quantum efficiency vs. current density for thedevices of the examples.

FIG. 7 shows the emission spectra at a current density (J) of 10 mA/cm²for the devices of the examples.

FIG. 8 shows the emission spectra for the device of Example 1 at variouscurrent densities.

FIG. 9 shows the emission spectra for the device of Example 4 at variouscurrent densities.

FIG. 10 shows the emission spectra for the device of Example 7 atvarious current densities.

DETAILED DESCRIPTION

The present invention provides organic light emitting devices having acombined emission from at least two emissive materials, a fluorescentblue emissive material and a phosphorescent emissive material. Thedevice may further comprise additional fluorescent or phosphorescentemissive materials. In preferred embodiments, the invention providesOLEDs having three different emissive materials—a red emissive material,a green emissive material and a blue emissive material. The inventionprovides a device architecture which is optimized for efficiency andlifetime by using a combination of fluorescent and phosphorescentemitters. Further, in preferred embodiments the device architectureprovides a high color-stability of the light emission over a wide rangeof currents or luminances. Further, the invention provides devices thatemit light from a combined emission with a high external quantumefficiency. In preferred embodiments the devices of the invention have amaximum external quantum efficiency of at least about 4.5%, or morepreferably at least about 5.0%, and still more preferably at least about6.0%.

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”) as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference in their entireties. Phosphorescence may be referred to asa “forbidden” transition because the transition requires a change inspin states, and quantum mechanics indicates that such a transition isnot favored. As a result, phosphorescence generally occurs in a timeframe exceeding at least 10 nanoseconds, and typically greater than 100nanoseconds. If the natural radiative lifetime of phosphorescence is toolong, triplets may decay by a non-radiative mechanism, such that nolight is emitted. Organic phosphorescence is also often observed inmolecules containing heteroatoms with unshared pairs of electrons atvery low temperatures. 2,2′-bipyridine is such a molecule. Non-radiativedecay mechanisms are typically temperature dependent, such that anorganic material that exhibits phosphorescence at liquid nitrogentemperatures typically does not exhibit phosphorescence at roomtemperature. But, as demonstrated by Baldo, this problem may beaddressed by selecting phosphorescent compounds that do phosphoresce atroom temperature. Representative emissive layers include doped orundoped phosphorescent organometallic materials such as disclosed inU.S. Pat. Nos. 6,303,238 and 6,310,360; U.S. Patent ApplicationPublication Nos. 2002-0034656; 2002-0182441; 2003-0072964; andWO-02/074015.

Generally, the excitons in an OLED are believed to be created in a ratioof about 3:1, i.e., approximately 75% triplets and 25% singlets. See,Adachi et al., “Nearly 100% Internal Phosphorescent Efficiency In AnOrganic Light Emitting Device,” J. Appl. Phys., 90, 5048 (2001), whichis incorporated by reference in its entirety. In many cases, singletexcitons may readily transfer their energy to triplet excited states via“intersystem crossing,” whereas triplet excitons may not readilytransfer their energy to singlet excited states. As a result, 100%internal quantum efficiency is theoretically possible withphosphorescent OLEDs. In a fluorescent device, the energy of tripletexcitons is generally lost to radiationless decay processes that heat-upthe device, resulting in much lower internal quantum efficiencies. OLEDsutilizing phosphorescent materials that emit from triplet excited statesare disclosed, for example, in U.S. Pat. No. 6,303,238, which isincorporated by reference in its entirety.

Phosphorescence may be preceded by a transition from a triplet excitedstate to an intermediate non-triplet state from which the emissive decayoccurs. For example, organic molecules coordinated to lanthanideelements often phosphoresce from excited states localized on thelanthanide metal. However, such materials do not phosphoresce directlyfrom a triplet excited state but instead emit from an atomic excitedstate centered on the lanthanide metal ion. The europium diketonatecomplexes illustrate one group of these types of species.

Phosphorescence from triplets can be enhanced over fluorescence byconfining, preferably through bonding, the organic molecule in closeproximity to an atom of high atomic number. This phenomenon, called theheavy atom effect, is created by a mechanism known as spin-orbitcoupling. Such a phosphorescent transition may be observed from anexcited metal-to-ligand charge transfer (MLCT) state of anorganometallic molecule such as tris(2-phenylpyridine)iridium(III).

As used herein, the term “triplet energy” refers to an energycorresponding to the highest energy feature discernable in thephosphorescence spectrum of a given material. The highest energy featureis not necessarily the peak having the greatest intensity in thephosphorescence spectrum, and could, for example, be a local maximum ofa clear shoulder on the high energy side of such a peak.

The term “organometallic” as used herein is as generally understood byone of ordinary skill in the art and as given, for example, in“Inorganic Chemistry” (2nd Edition) by Gary L. Miessler and Donald A.Tarr, Prentice Hall (1998). Thus, the term organometallic refers tocompounds which have an organic group bonded to a metal through acarbon-metal bond. This class does not include per se coordinationcompounds, which are substances having only donor bonds fromheteroatoms, such as metal complexes of amines, halides, pseudohalides(CN, etc.), and the like. In practice organometallic compounds generallycomprise, in addition to one or more carbon-metal bonds to an organicspecies, one or more donor bonds from a heteroatom. The carbon-metalbond to an organic species refers to a direct bond between a metal and acarbon atom of an organic group, such as phenyl, alkyl, alkenyl, etc.,but does not refer to a metal bond to an “inorganic carbon,” such as thecarbon of CN or CO.

FIG. 1 shows an organic light emitting device 100. The figures are notnecessarily drawn to scale. Device 100 may include a substrate 110, ananode 115, a hole injection layer 120, a hole transport layer 125, anelectron blocking layer 130, an emissive region 135, a hole blockinglayer 140, an electron transport layer 145, an electron injection layer150, a protective layer 155, and a cathode 160. Cathode 160 is acompound cathode having a first conductive layer 162 and a secondconductive layer 164. Device 100 may be fabricated by depositing thelayers described, in order.

Substrate 110 may be any suitable substrate that provides desiredstructural properties. Substrate 110 may be flexible or rigid. Substrate110 may be transparent, translucent or opaque. Plastic and glass areexamples of preferred rigid substrate materials. Plastic and metal foilsare examples of preferred flexible substrate materials. Substrate 110may be a semiconductor material in order to facilitate the fabricationof circuitry. For example, substrate 110 may be a silicon wafer uponwhich circuits are fabricated, capable of controlling OLEDs subsequentlydeposited on the substrate. Other substrates may be used. The materialand thickness of substrate 110 may be chosen to obtain desiredstructural and optical properties.

Anode 115 may be any suitable anode that is sufficiently conductive totransport holes to the organic layers. The material of anode 115preferably has a work function higher than about 4 eV (a “high workfunction material”). Preferred anode materials include conductive metaloxides, such as indium tin oxide (ITO) and indium zinc oxide (IZO),aluminum zinc oxide (AlZnO), and metals. Anode 115 (and substrate 110)may be sufficiently transparent to create a bottom-emitting device. Apreferred transparent substrate and anode combination is commerciallyavailable ITO (anode) deposited on glass or plastic (substrate). Aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. Nos. 5,844,363 and 6,602,540 B2, which are incorporated byreference in their entireties. Anode 115 may be opaque and/orreflective. A reflective anode 115 may be preferred for sometop-emitting devices, to increase the amount of light emitted from thetop of the device. The material and thickness of anode 115 may be chosento obtain desired conductive and optical properties. Where anode 115 istransparent, there may be a range of thickness for a particular materialthat is thick enough to provide the desired conductivity, yet thinenough to provide the desired degree of transparency. Other anodematerials and structures may be used.

Hole transport layer 125 may include a material capable of transportingholes. Hole transport layer 130 may be intrinsic (undoped), or doped.Doping may be used to enhance conductivity. α-NPD and TPD are examplesof intrinsic hole transport layers. An example of a p-doped holetransport layer is m-MTDATA doped with F₄-TCNQ at a molar ratio of 50:1,as disclosed in United States Patent Application Publication No.2003-02309890 to Forrest et al., which is incorporated by reference inits entirety. Other hole transport layers may be used.

Emissive region 135 is comprised of at least two adjacent emissivelayers, each of which includes an organic material capable of emittinglight when a current is passed between anode 115 and cathode 160. Atleast one of the emissive materials should be a phosphorescent emissivematerial, and at least on of the emissive materials should be afluorescent emissive material. Phosphorescent materials are preferredbecause of the higher luminescent efficiencies associated with suchmaterials. An emissive layer may comprise other materials, such asdopants that tune the emission of the emissive material. The emissiveregion includes a plurality of emissive materials capable of, incombination, emitting a desired spectrum of light. Examples ofphosphorescent emissive materials include Ir(Ppy)₃. Examples offluorescent emissive materials include DCM and DMQA. The emissive layersof the emissive region 135 may also comprise a host material capable oftransporting electrons and/or holes, doped with an emissive materialthat may trap electrons, holes, and/or excitons, such that excitonsrelax from the emissive material via a photoemissive mechanism. Examplesof host materials include Alq₃, CBP and mCP. Examples of emissive andhost materials are disclosed in U.S. Pat. No. 6,303,238 to Thompson etal., which is incorporated by reference in its entirety. Emissivematerial may be included in an emissive layer in a number of ways. Forexample, an emissive small molecule may be incorporated into a polymer.This may be accomplished by several ways: by doping the small moleculeinto the polymer either as a separate and distinct molecular species; orby incorporating the small molecule into the backbone of the polymer, soas to form a co-polymer; or by bonding the small molecule as a pendantgroup on the polymer. Other emissive layer materials and structures maybe used. For example, a small molecule emissive material may be presentas the core of a dendrimer.

Many useful emissive materials include one or more ligands bound to ametal center. A ligand may be referred to as “photoactive” if itcontributes directly to the photoactive properties of an organometallicemissive material. A “photoactive” ligand may provide, in conjunctionwith a metal, the energy levels from which and to which an electronmoves when a photon is emitted. Other ligands may be referred to as“ancillary.” Ancillary ligands may modify the photoactive properties ofthe molecule, for example by shifting the energy levels of a photoactiveligand, but ancillary ligands do not directly provide the energy levelsinvolved in light emission. A ligand that is photoactive in one moleculemay be ancillary in another. These definitions of photoactive andancillary are intended as non-limiting theories.

Electron transport layer 145 may include a material capable oftransporting electrons. Electron transport layer 145 may be intrinsic(undoped), or doped. Doping may be used to enhance conductivity. Alq₃ isan example of an intrinsic electron transport layer. An example of ann-doped electron transport layer is BPhen doped with Li at a molar ratioof 1:1, as disclosed in United States Patent Application Publication No.2003-02309890 to Forrest et al., which is incorporated by reference inits entirety. Other electron transport layers may be used.

The charge carrying component of the electron transport layer may beselected such that electrons can be efficiently injected from thecathode into the LUMO (Lowest Unoccupied Molecular Orbital) energy levelof the electron transport layer. The “charge carrying component” is thematerial responsible for the LUMO energy level that actually transportselectrons. This component may be the base material, or it may be adopant. The LUMO energy level of an organic material may be generallycharacterized by the electron affinity of that material and the relativeelectron injection efficiency of a cathode may be generallycharacterized in terms of the work function of the cathode material.This means that the preferred properties of an electron transport layerand the adjacent cathode may be specified in terms of the electronaffinity of the charge carrying component of the ETL and the workfunction of the cathode material. In particular, so as to achieve highelectron injection efficiency, the work function of the cathode materialis preferably not greater than the electron affinity of the chargecarrying component of the electron transport layer by more than about0.75 eV, more preferably, by not more than about 0.5 eV. Similarconsiderations apply to any layer into which electrons are beinginjected.

Cathode 160 may be any suitable material or combination of materialsknown to the art, such that cathode 160 is capable of conductingelectrons and injecting them into the organic layers of device 100.Cathode 160 may be transparent or opaque, and may be reflective. Metalsand metal oxides are examples of suitable cathode materials. Cathode 160may be a single layer, or may have a compound structure. FIG. 1 shows acompound cathode 160 having a thin metal layer 162 and a thickerconductive metal oxide layer 164. In a compound cathode, preferredmaterials for the thicker layer 164 include ITO, IZO, and othermaterials known to the art. U.S. Pat. Nos. 5,703,436, 5,707,745,6,548,956 B2 and 6,576,134 B2, which are incorporated by reference intheir entireties, disclose examples of cathodes including compoundcathodes having a thin layer of metal such as Mg:Ag with an overlyingtransparent, electrically-conductive, sputter-deposited ITO layer. Thepart of cathode 160 that is in contact with the underlying organiclayer, whether it is a single layer cathode 160, the thin metal layer162 of a compound cathode, or some other part, is preferably made of amaterial having a work function lower than about 4 eV (a “low workfunction material”). Other cathode materials and structures may be used.

Blocking layers may be used to reduce the number of charge carriers(electrons or holes) and/or excitons that leave the emissive layer. Anelectron blocking layer 130 may be disposed between emissive layer 135and the hole transport layer 125, to block electrons from leavingemissive layer 135 in the direction of hole transport layer 125.Similarly, a hole blocking layer 140 may be disposed between emissivelayer 135 and electron transport layer 145, to block holes from leavingemissive layer 135 in the direction of electron transport layer 145.Blocking layers may also be used to block excitons from diffusing out ofthe emissive layer. The theory and use of blocking layers is describedin more detail in U.S. Pat. No. 6,097,147 and United States PatentApplication Publication No. 2003-02309890 to Forrest et al., which areincorporated by reference in their entireties.

As used herein, and as would be understood by one skilled in the art,the term “blocking layer” means that the layer provides a barrier thatsignificantly inhibits transport of charge carriers and/or excitonsthrough the device, without suggesting that the layer necessarilycompletely blocks the charge carriers and/or excitons. The presence ofsuch a blocking layer in a device may result in substantially higherefficiencies as compared to a similar device lacking a blocking layer.Also, a blocking layer may be used to confine emission to a desiredregion of an OLED.

Generally, injection layers are comprised of a material that may improvethe injection of charge carriers from one layer, such as an electrode oran organic layer, into an adjacent organic layer. Injection layers mayalso perform a charge transport function. In device 100, hole injectionlayer 120 may be any layer that improves the injection of holes fromanode 115 into hole transport layer 125. CuPc is an example of amaterial that may be used as a hole injection layer from an ITO anode115, and other anodes. In device 100, electron injection layer 150 maybe any layer that improves the injection of electrons into electrontransport layer 145. LiF/Al is an example of a material that may be usedas an electron injection layer into an electron transport layer from anadjacent layer. Other materials or combinations of materials may be usedfor injection layers. Depending upon the configuration of a particulardevice, injection layers may be disposed at locations different thanthose shown in device 100. More examples of injection layers areprovided in U.S. patent application Ser. No. 09/931,948 to Lu et al.,which is incorporated by reference in its entirety. A hole injectionlayer may comprise a solution deposited material, such as a spin-coatedpolymer, e.g., PEDOT:PSS, or it may be a vapor deposited small moleculematerial, e.g., CuPc or MTDATA.

A hole injection layer (HIL) may planarize or wet the anode surface soas to provide efficient hole injection from the anode into the holeinjecting material. A hole injection layer may also have a chargecarrying component having HOMO (Highest Occupied Molecular Orbital)energy levels that favorably match up, as defined by theirherein-described relative ionization potential (IP) energies, with theadjacent anode layer on one side of the HIL and the hole transportinglayer on the opposite side of the HIL. The “charge carrying component”is the material responsible for the HOMO energy level that actuallytransports holes. This component may be the base material of the HIL, orit may be a dopant. Using a doped HIL allows the dopant to be selectedfor its electrical properties, and the host to be selected formorphological properties such as wetting, flexibility, toughness, etc.Preferred properties for the HIL material are such that holes can beefficiently injected from the anode into the HIL material. Inparticular, the charge carrying component of the HIL preferably has anIP not more than about 0.7 eV greater that the IP of the anode material.More preferably, the charge carrying component has an IP not more thanabout 0.5 eV greater than the anode material. Similar considerationsapply to any layer into which holes are being injected. HIL materialsare further distinguished from conventional hole transporting materialsthat are typically used in the hole transporting layer of an OLED inthat such HIL materials may have a hole conductivity that issubstantially less than the hole conductivity of conventional holetransporting materials. The thickness of the HIL of the presentinvention may be thick enough to help planarize or wet the surface ofthe anode layer. For example, an HIL thickness of as little as 10 nm maybe acceptable for a very smooth anode surface. However, since anodesurfaces tend to be very rough, a thickness for the HIL of up to 50 nmmay be desired in some cases.

A protective layer may be used to protect underlying layers duringsubsequent fabrication processes. For example, the processes used tofabricate metal or metal oxide top electrodes may damage organic layers,and a protective layer may be used to reduce or eliminate such damage.In device 100, protective layer 155 may reduce damage to underlyingorganic layers during the fabrication of cathode 160. Preferably, aprotective layer has a high carrier mobility for the type of carrierthat it transports (electrons in device 100), such that it does notsignificantly increase the operating voltage of device 100. CuPc, BCP,and various metal phthalocyanines are examples of materials that may beused in protective layers. Other materials or combinations of materialsmay be used. The thickness of protective layer 155 is preferably thickenough that there is little or no damage to underlying layers due tofabrication processes that occur after organic protective layer 160 isdeposited, yet not so thick as to significantly increase the operatingvoltage of device 100. Protective layer 155 may be doped to increase itsconductivity. For example, a CuPc or BCP protective layer 160 may bedoped with Li. A more detailed description of protective layers may befound in U.S. patent application Ser. No. 09/931,948 to Lu et al., whichis incorporated by reference in its entirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210,an cathode 215, an emissive region 220, a hole transport layer 225, andan anode 230. Device 200 may be fabricated by depositing the layersdescribed, in order. Because the most common OLED configuration has acathode disposed over the anode, and device 200 has cathode 215 disposedunder anode 230, device 200 may be referred to as an “inverted” OLED.Materials similar to those described with respect to device 100 may beused in the corresponding layers of device 200. FIG. 2 provides oneexample of how some layers may be omitted from the structure of device100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of non-limiting example, and it is understood that embodiments ofthe invention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly limiting. For example, in device 200, holetransport layer 225 transports holes and injects holes into emissiveregion 220, and may be described as a hole transport layer or a holeinjection layer. In one embodiment, an OLED may be described as havingan “organic layer” disposed between a cathode and an anode. This organiclayer may comprise a single layer, or may further comprise multiplelayers of different organic materials as described, for example, withrespect to FIGS. 1 and 2.

Structures and materials not specifically described may also be used,such as OLEDs comprised of polymeric materials (PLEDs) such as disclosedin U.S. Pat. No. 5,247,190, Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove out-coupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al., and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. patent application Ser. No. 10/233,470, which is incorporated byreference in its entirety. Other suitable deposition methods includespin coating and other solution based processes. Solution basedprocesses are preferably carried out in nitrogen or an inert atmosphere.For the other layers, preferred methods include thermal evaporation.Preferred patterning methods include deposition through a mask, coldwelding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819,which are incorporated by reference in their entireties, and patterningassociated with some of the deposition methods such as ink-jet and OVJD.Other methods may also be used. The materials to be deposited may bemodified to make them compatible with a particular deposition method.For example, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processibility than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution processing.

The molecules disclosed herein may be substituted in a number ofdifferent ways without departing from the scope of the invention. Forexample, substituents may be added to a compound having three bidentateligands, such that after the substituents are added, one or more of thebidentate ligands are linked together to form, for example, atetradentate or hexadentate ligand. Other such linkages may be formed.It is believed that this type of linking may increase stability relativeto a similar compound without linking, due to what is generallyunderstood in the art as a “chelating effect.”

Devices fabricated in accordance with embodiments of the invention maybe incorporated into a wide variety of consumer products, including flatpanel displays, computer monitors, televisions, billboards, lights forinterior or exterior illumination and/or signaling, heads up displays,fully transparent displays, flexible displays, laser printers,telephones, cell phones, personal digital assistants (PDAs), laptopcomputers, digital cameras, camcorders, viewfinders, micro-displays,vehicles, a large area wall, theater or stadium screen, or a sign.Various control mechanisms may be used to control devices fabricated inaccordance with the present invention, including passive matrix andactive matrix. Many of the devices are intended for use in a temperaturerange comfortable to humans, such as 18 degrees C. to 30 degrees C., andmore preferably at room temperature (20-25 degrees C.).

The materials and structures described herein may have applications indevices other than OLEDs. For example, other optoelectronic devices suchas organic solar cells and organic photodetectors may employ thematerials and structures. More generally, organic devices, such asorganic transistors, may employ the materials and structures.

The devices of the present invention comprise an emissive region,wherein the emissive region has at least two adjacent emissive layers, afirst emissive layer and a second emissive layer. In one embodiment,each emissive layer comprises a host material and at least one emissivedopant material. The first emissive layer is comprised of a first hostmaterial and a first emissive material, and the second emissive layer iscomprised of a second host material and a second emissive material. Thehost materials for the two or more emissive layers may be the same ormay be different, however, in preferred embodiments the hosts comprisedifferent materials. In preferred embodiments, the emissive regionfurther comprises a spacer layer between the two adjacent emissivelayers.

In a preferred embodiment of the invention, the emissive region of thedevices of the present invention comprises at least one blue fluorescentemissive material. To date, phosphorescent blue emitters have generallydisplayed poor operational stability in OLEDs. Fluorescent blue emittersare selected to have good operational lifetimes in OLEDs. The emissivelayer that comprises the fluorescent blue emissive material may notfurther be comprised of a phosphorescent emissive material. It isbelieved that the low triplet energy of the fluorescent blue emissivematerial would quench the emission from the phosphorescent material ifthe emissive materials were mixed within the same emissive layer. Thus,the fluorescent blue emissive material will be present in a separateemissive layer from the phosphorescent emissive materials. In certainembodiments, the emissive layer comprising the fluorescent blue emissivematerial will further comprise additional fluorescent emissivematerials.

The invention provides efficient white-emitting or multi-coloredemitting OLEDs. For white emitting devices, the combined emission of theemitting materials gives a white emission from the device. For preferredwhite-emitting devices, the two or more emissive dopants are selected sothat the combined emission from the device has a CIE range betweenX=0.37±0.07 and Y=0.37±0.07. More preferably, the CIE coordinates areX=0.35±0.05, and Y=0.35±0.05 and even more preferably X=0.33±0.02,Y=0.33±0.02. The term “multicolored” refers to the emission from adevice that results from two or more different emissive dopants thateach have different emissive spectra. Although a high CRI value may bepreferred for certain lighting applications, the devices of the presentinvention may be used to produce a light source that provides othercolors as well. In preferred embodiments, the devices of the presentinvention are capable of achieving an external quantum efficiency of atleast about 6%. The devices of the present invention preferably havelifetimes of greater than about 200 hours at a current density of 40mA/cm². More preferably the devices have a lifetime of greater thanabout 400 hours and still more preferably have a lifetime greater thanabout 1000 hours.

In a preferred embodiment, each emissive layer comprises a differenthost material and at least one emissive dopant material. In thisembodiment, the first host material and the second host material aredifferent chemical compounds. In preferred embodiments, the differenthost materials of the adjacent emissive layers may be selected so as toprovide a “heterojunction” between the emissive layers that localizesrecombination of electrons and holes at or near the heterojunction. Theheterojunction may provide a narrow zone for the recombination ofelectrons and holes which does not shift significantly as a function ofthe drive current applied to the device. Recombination occurs primarilyat or near the heterojunction and the resulting excitons may diffuse toboth sides of the heterojunction. As the emissive layers that form theheterojunction contain different emissive dopants, the device emissionspectrum contains the electroluminescence from multiple emissivematerials. Thus, this device structure may provide a highcolor-stability of the light emission over a wide range of currents orluminances.

In one embodiment, the heterojunction between the adjacent emissivelayers of the emissive region provides an electron injection barrier. Inthis case, electrons are inhibited from crossing the heterojunctionbetween the emissive layers. Electrons, originating at the cathode, aretransported through the emissive layer on the cathode-side of thejunction. The host materials of the emissive layers that form theheterojunction are selected such that electrons, upon reaching theheterojunction, are inhibited from transport into or through theemissive layer on the anode-side of the junction. This may be achievedby selecting the different host materials of the emissive layers thatform the heterojunction based on the relative LUMOs of the two hostmaterials. The host material of the emissive layer on the anode-side ofthe heterojunction is selected to have a LUMO that is higher than thehost material of the emissive layer on the cathode-side of theheterojunction. The difference in the LUMO levels of the first andsecond emissive layer should preferably be at least 0.25 eV, morepreferably at least 0.3 eV, and even more preferably at least 0.7 eV. Inanother embodiment, the emissive layer on the anode-side of the junctionmay have a lower electron mobility than the emissive layer on thecathode-side of the junction.

In another embodiment of the invention, the heterojunction between theadjacent emissive layers of the emissive region provides a holeinjection barrier. In this case, holes are inhibited from crossing theheterojunction between the emissive layers. Holes, originating at theanode, are transported through the emissive layer on the anode-side ofthe junction. The host materials of the emissive layers that form theheterojunction are selected such that holes, upon reaching theheterojunction, are inhibited from transport into or through theemissive layer on the cathode-side of the junction. This may be achievedby selecting the different host materials of the emissive layers thatform the heterojunction based on the relative HOMOs of the two hostmaterials. The host material of the emissive layer on the cathode-sideof the heterojunction is selected to have a HOMO that is lower than thehost material of the emissive layer on the anode-side of theheterojunction. The difference in the HOMO levels of the first andsecond emissive layer should preferably be at least 0.25 eV, morepreferably at least 0.3 eV, and even more preferably at least 0.7 eV. Inanother embodiment, the emissive layer on the cathode-side of thejunction may have a lower hole mobility than the emissive layer on thecathode-side of the junction.

In a further embodiment of the invention, the heterojunction may displayany combination of the properties that work to localize recombination ator near the heterojunction. For example, the heterojunction may provideboth an electron injection barrier and a hole injection barrier.

The electronic properties of the host materials (for example, when thehost is an undoped thin layer) are believed be a good approximation ofthe electronic properties of the doped layer. Thus, for example, theelectron mobility of the doped layer is approximated by the electronmobility of the host material. In the case of phosphorescent dopants,which are generally present at a concentration of about 6%, the dopantitself may contribute to the electronic properties of the doped layer.In the cases where the phosphorescent dopant does contribute to theelectronic properties of the layer, it may be useful to describe theproperties (e.g., LUMO, HOMO, electron mobility, hole mobility, etc.) asbeing a property of the doped layer.

In the preferred embodiments in which the emissive region comprises aspacer region between the blue fluorescent emissive layer and theadjacent phosphorescent emissive layer, the heterojunction as describedabove will be created by the interface between the spacer layer and oneof the adjacent emissive layers. In further preferred embodiments, theheterojunction is provided by the interface between the spacer layer andthe blue fluorescent emissive layer.

The emissive region may be comprised of additional emissive layers. Inanother embodiment of the invention, the emissive region has threeemissive layers. Two of the adjacent emissive layers form theheterojunction as described above. An additional emissive layer may beadjacent to either the emissive layer on the cathode-side of thejunction or the emissive layer on the anode-side of the heterojunction.The additional layer may be comprised of a host material and an emissivedopant. Preferably, the emissive dopant emits at a different frequencythan the emissive dopants of the two emissive layers that form theheterojunction. The host material for this layer may be the same ordifferent from the host materials of the two emissive layers that formthe heterojunction, although in preferred embodiments the host materialwill be will be the same as an adjacent emissive layer.

In one embodiment of the invention, the emissive region comprises threeemissive material—a fluorescent blue emitter, a fluorescent greenemitter, and a phosphorescent red emitter. The fluorescent blue emitterand the fluorescent green emitter may be combined in the same emissivelayer or may be in separated emissive layers. The phosphorescent redemissive material will be in a separate emissive layer within theemissive region. In preferred embodiments, each emissive layer willcomprise the emissive material(s) in a host material. The host materialmay be the same or may be different for each of the emissive layers. Inpreferred embodiments, the host material for the fluorescent emissivematerials is different than the host for the phosphorescent emissivematerial. In particularly preferred embodiments, the interface betweenthe host material for the fluorescent emissive materials and the hostfor the phosphorescent emissive material forms a heterojunction asdescribed above.

In another embodiment of the invention, the emissive region comprisesthree emissive material—a fluorescent blue emitter, a phosphorescentgreen emitter, and a phosphorescent red emitter. The phosphorescentgreen emitter and the phosphorescent red emitter may be combined in thesame emissive layer or may be in separated emissive layers. Thefluorescent blue emissive material will be in a separate emissive layerwithin the emissive region. In preferred embodiments, each emissivelayer will comprise the emissive material(s) in a host material. Thehost material may the same or may be different for each of the emissivelayers. In preferred embodiments, the host material for the fluorescentemissive material is different than the host for the phosphorescentemissive materials. In particularly preferred embodiments, the interfacebetween the host material for the fluorescent emissive material and thehost for the phosphorescent emissive materials forms a heterojunction asdescribed above.

In particularly preferred embodiments of the present invention, theemissive region further comprises a spacer layer between the hostmaterial for the fluorescent emissive material(s) and the host for thephosphorescent emissive material(s). As noted above, fluorescentmaterials generally have low triplet energies and, therefore, may quenchthe triplet emission from the phosphorescent emissive materials.Further, the host for the fluorescent material(s) may also have a lowenough triplet energy such that it would cause quenching. The spacerlayer provides a buffer that separates the fluorescent blue emissivematerial from the phosphorescent emissive material(s). The spacer layerneed only be thick enough to inhibit quenching. Further, the spacerlayer should be sufficiently thin that the excitons formed at or nearthe heterojunction, which is formed at the interface between the spacerand one or both adjacent emissive layers, can diffuse to the otheradjacent emissive layer. Further, a thin layer has a smaller effect onthe drive voltage of the device. Thus, in preferred embodiments thespacer layer is between about 10 Å to 50 Å thick, and in particularlypreferred embodiments the spacer layer is between about 10 Å to 25 Åthick. The spacer layer preferably comprises a material that has ahigher triplet energy than the triplet energy of the phosphorescentemissive material(s). In preferred embodiments the spacer layer iscomprised of the host material for the phosphorescent emissivematerial(s).

Fluorescent blue emitters preferably emit visible light with a peakwavelength of less than about 470 nm, and more preferably less thanabout 450 nm. In preferred embodiments, the fluorescent blue emitteremits light having CIE coordinates of about x=0.15±0.02, and abouty≦0.15. Preferred fluorescent blue emitters include polyaromaticcompounds such as 9,10-di(2-naphthylantracene), perylenes, phenylenes,and fluorenes. Preferred fluorescent blue emitters may be found in C. H.Chen, J. Shi, and C. W. Tang, “Recent Developments in Molecular OrganicElectroluminescent Materials,” Macromol. Symp. 125, pp. 1-48 (1997) andthe references cited therein; L. S. Hung and C. H. Chen, “Recentprogress of molecular organic electroluminescent materials and devices,”Mat. Sci and Eng. R, 39 (2002), pp. 143-222 and the references citedtherein, each of which is incorporated herein by reference in theirentirety.

Particularly preferred fluorescent blue emitters include arylpyrenes asdescribed in the co-pending application entitled “Arylpyrene Compounds,”U.S. patent application Ser. No. 11/097,352, filed Apr. 4, 2005, whichis incorporated herein by reference in its entirety. The preferredarylpyrenes can be generally described by formula IV: The arylpyrenecompound can be generally described by formula IV:

In one embodiment, each R group is the same substituent. Substituentsinclude, for example, R′, O—R′, N(R′)₂, SR′, C(O)R′, C(O)OR′, C(O)NR′₂,CN, CF₃, NO₂, SO₂, SOR′, SO₃R′, or halo. Any two substituents onadjacent ring atoms can optionally form a 5- or 6-member cyclic group,such that the 1-, 3-, 6-, or 8-aryl group is a fused ring group, such asa naphthyl or phenanthryl group. The fused ring may be optionallysubstituted with one or more substituents, as defined above. Each R′ isindependently H, alkyl, alkenyl, alkynyl, heteroalkyl, aralkyl, aryl, orheteroaryl. In one embodiment, the R group is a straight, branched, orcyclic C₁₋₁₀ alkyl group or a fused ring group. It is believed thatalkyl substituents improve the solubility of the compound, therebymaking the compound better suited for deposition by solution process.Branched alkyl substituents may be particularly effective at increasingsolubility.

In one embodiment, the arylpyrene compound is selected from the groupconsisting of formulas I, II, and III.

In each of formulas I, II, and III, each of the 1-, 3-, 6-, and 8-arylgroups may be optionally and independently substituted with one or moresubstituents. Thus, in one embodiment, each aryl group, e.g., thenaphthyl group in formula I, is unsubstituted. In another embodiment,each aryl group may have unique substituent(s). For example, the arylgroups may be independently substituted with a straight, branched, orcyclic C₁₋₁₀ alkyl group or a fused ring group.

Other particularly preferred blue fluorescent emitters includearylenevinylene compounds as described in U.S. Pat. Nos. 5,121,029 and5,130,603, which are incorporated herein by reference in their entirety.The arylenevinylene compounds may have the general formula:

wherein R¹ and R² are each an alkyl, cyclohexyl, alkoxy, cyano or aryl,R³ and R⁴ are a heterocyclic or aryl and Ar is an divalent aromaticgroup. In preferred embodiments, R¹, R², R³ and R⁴ are independentlyselected from an aromatic group.

Other preferred arylenevinylene compounds may have the general formulas:X—CH═CH—Y—CH═CH—Z and X—C═CH—Zwherein X and Z are independently an aromatic group and Y is a divalentaromatic group. In preferred embodiments, X and Z independentlyrepresent an unsubstituted phenyl group or an alkyl-substituted phenylgroup having an alkyl group with 1 to 4 carbon atoms and Y represents anunsubstituted phenylene group or a mono alkyl-substituted phenylenegroup having an alkyl group with 1 to 4 carbon atoms.

In one embodiment of the invention the blue fluorescent material may bepresent as a neat layer. In preferred embodiments of the invention, theblue fluorescent material would be present as a dopant in a hostmaterial. Preferred host materials for the blue fluorescent emittersinclude, for example, anthracenes and carbazoles. Anthracene hostsinclude, but are not limited to 9,10-di(2-naphthyl)anthracene (ADN) and9,10-di[3-(N-carbazole)phenyl]-anthracene (B3CPA). In a preferredembodiment, the anthracene host is ADN. The preferred host materials(such as the anthracene hosts) are good electron transporting materials.Thus, in preferred embodiments the emissive layer comprising thefluorescent blue material is on the cathode-side of the heterojunction.

Green emitters preferably emit visible light with a peak wavelengthbetween about 500 nm to about 550 nm. In preferred embodiments, thegreen emitter emits light having CIE coordinates of about x=0.30, andabout y=0.60. Preferred fluorescent green emitters include the coumarindyes, julolidine dyes, quinacridones, and imidazolinones. A discussionof preferred fluorescent emissive materials may be found in C. H. Chen,J. Shi, and C. W. Tang, “Recent Developments in Molecular OrganicElectroluminescent Materials,” Macromol. Symp. 125, pp. 1-48 (1997) andthe references cited therein; L. S. Hung and C. H. Chen, “Recentprogress of molecular organic electroluminescent materials and devices,”Mat. Sci and Eng. R, 39 (2002), pp. 143-222 and the references citedtherein, each of which is incorporated herein by reference in theirentirety. Preferred green phosphorescent emitters include derivatives of2-phenylpyridine-iridium complexes, such as Ir(Ph-ppy)₃. Preferredphosphorescent green emitters may be found in Baldo, M. A., Thompson, M.E. & Forrest, S. R. High efficiency fluorescent organic light-emittingdevices using a phosphorescent sensitizer, Nature 403, 750-753 (2000);and in U.S. Pat. No. 6,830,828, each of which is incorporated herein byreference in their entirety.

Phosphorescent red emitters preferably emit visible light with a peakwavelength between about 600 nm to about 630 nm. In preferredembodiments, the red emitter emits light having CIE coordinates of aboutx=0.67±0.03, and about y=0.33±0.03. Preferred phosphorescent redemitters are derivatives of 2-phenylpyridine-iridium complexes, such asIr(Me-pq)₂(acac). Preferred phosphorescent red emitters may be found inU.S. Pat. Nos. 6,835,469 and 6,830,828, each of which is incorporatedherein by reference in their entirety.

The HOMO and LUMO energy levels for organic materials to be used inOLEDs have been estimated in several ways. The two common methods forestimating HOMO levels are solution electrochemistry and ultravioletphotoelectron spectroscopy (UPS). The two common methods for estimatingLUMO levels are solution electrochemistry and inverse photoemissionspectroscopy. These energies are useful for predicting the interactionsdescribed herein between the emissive material and the host material ofthe emissive layer. Additionally, the alignment of the HOMO and LUMOlevels between adjacent layers will control the passage of hole andelectrons between the two layers.

The most common method for determining oxidation and reductionpotentials is cyclic voltammetry. A simple description of this processis as follows. The unknown is dissolved along with a high concentrationof electrolyte. Electrodes are inserted and the voltage scanned ineither the positive or negative direction (depending on whether anoxidation or reduction is being performed). The presence of a redoxreaction is indicated by current flowing through the cell. The voltagescan is then reversed and the redox reaction is reversed. If the areasof the two redox waves are the same the process was reversible. Thepotential at which these events occur give the value of the reduction oroxidation potential relative to a reference. The reference can be anexternal electrode, such as Ag/AgCl or SCE, or it can be an internalone, such as ferrocene, which has a known oxidation potential. Thelatter is often preferred for organic solvents, since the commonreference electrodes are water based. Although this is a solutionprocess, in contrast to the solid state OLED, and the reference may behard to adjust to give values relative to vacuum, the method is good forgiving relative numbers. One useful parameter that may come fromelectrochemical measurement is the carrier gap. If both the reductionand oxidation are reversible, one can determine the energy differencebetween the hole and the electron (i.e. taking an electron out of theHOMO versus putting one into the LUMO). This value is important todetermine the LUMO energy from a well defined HOMO energy. If either ofthe redox processes are not reversible, the carrier gap cannot bedetermined by this method.

The preferred method to estimate HOMO energies in the solid state isUPS. This is a photoelectric measurement, where the solid is irraditatedwith UV photons. The energy of the photons is gradually increased untilphotogenerated electrons are observed. The onset of ejected electronsgives the energy of the HOMO. The photons at that energy have justenough energy to eject an electron form the top of the filled levels.Inverse photoemission involves pre-reducing the sample and then probingthe filled states to estimate the LUMO energies. The best acceptedmethod for determining HOMO energies is UPS, which gives values in eVrelative to vacuum. This is the binding energy for the electron.

Another important parameter is the optical gap. This value is typicallydetermined from the intersection of the normalized absorption andemission spectra. For a molecule that has very little structuralrearrangement in the excited state, the gap between the absorption andemission λ_(max) values is rather small and this intersection energy isa good estimate of the optical gap (the 0-0 transition energy). This isoften held to be the HOMO-LUMO gap. In some cases, this can be a poorestimation if the shift between the absorption and emission maxima islarge (Stokes shift), such that the optical gap is hard to determine. Ifthere is a structural rearrangement in the excited state or theabsorption that is measured is not the one for the lowest energy excitedstate, then there can be a large error. Alternatively, the edge of theabsorption or emission bands may be used to estimate the optical gap. Insome cases, this is a poor estimate. When the optical gap is used toestimate LUMO energies from a measured HOMO energy, it is most usefulfor a well behaved molecule (i.e. a small Stokes shift), where it isclose to the HOMO-LUMO gap. Even in this case, a carrier gap may be abetter estimate and may be larger than the optical gap. If one isconcerned about exciton blocking, the edge of the absorption band ismore useful, as this will give the energy below which excitons will notbe efficiently trapped. That is to say, if an exciton of lower energythan the band edge of the materials approaches a layer with a higherenergy absorption edge the likelihood that the exciton will betransferred into this material is low. For molecules emitting fromtriplet excited states, the absorption edge is the best estimate, sincethe intersystem crossing may lead to a very large Stokes shift.

It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. For example, many of the materials and structures describedherein may be substituted with other materials and structures withoutdeviating from the spirit of the invention. It is understood thatvarious theories as to why the invention works are not intended to belimiting. For example, theories relating to charge transfer are notintended to be limiting.

MATERIAL DEFINITIONS

As used herein, abbreviations refer to materials as follows:

-   CBP: 4,4′-N,N-dicarbazole-biphenyl-   m-MTDATA 4,4′,4″-tris(3-methylphenylphenlyamino)triphenylamine-   Alq₃: 8-tris-hydroxyquinoline aluminum-   Bphen: 4,7-diphenyl-1,10-phenanthroline-   n-BPhen: n-doped BPhen (doped with lithium)-   F₄-TCNQ: tetrafluoro-tetracyano-quinodimethane-   p-MTDATA: p-doped m-MTDATA (doped with F₄-TCNQ)-   Ir(ppY)₃: tris(2-phenylpyridine)-iridium-   Ir(ppz)₃: tris(1-phenylpyrazoloto,N,C(2′)iridium(III)-   BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline-   TAZ: 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole-   CuPc: copper phthalocyanine.-   ITO: indium tin oxide-   NPD: N,N′-diphenyl-N—N′-di(1-naphthyl)-benzidine-   TPD: N,N′-diphenyl-N—N′-di(3-toly)-benzidine-   BAlq:    aluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate-   mCP: 1,3-N,N-dicarbazole-benzene-   DCM: 4-(dicyanoethylene)-6-(4-dimethylaminostyryl-2-methyl)-4H-pyran-   DMQA: N,N′-dimethylquinacridone-   PEDOT:PSS: an aqueous dispersion of poly(3,4-ethylenedioxythiophene)    with polystyrenesulfonate (PSS)-   Ir(4,6-F₂ppy)₂(BPz₄)    iridium(III)bis(2-(4,6-difluorphenyl)pyridinato-N,    C²)η²-N,N′-(tetrakis(1-pyrazolyl)borate)-   Ir(pq)₂(acac)    (2,4-pentanedionato-κO,κO′)bis[2-(2-quinolinyl-κN)phenyl-κC]iridium-   Ir(Me-pq)₂(acac)    bis[2-(3-methyl-2-quinolinyl-κN)phenyl-κC](2,4-pentanedionato-κO,κO′)iridium-   Ir(F₂CNppy)₂(pic)    Bis[2-(4,6-difluoro-5-cyanophenyl)pyridine]Iridium(III)picolinate-   Ir(Ph-ppy)₃ Iridium(III)tris-[2-(3-biphenyl)pyridine]-   TPBi 2,2′,2″-(1,3,5-benzenetriyl)tris-(1-phenyl-1H-benzimidazole)-   DTBD    2,2′-dimethyl-N,N,N′,N′-tetraphenyl-[1,1′-biphenyl]-4,4′-diamine-   BFD47 1,3,6,8-tetra(2-naphthyl)pyrene-   ADN: 9, 10-di-(2-naphthyl)anthracene

EXPERIMENTAL

Specific representative embodiments of the invention will now bedescribed, including how such embodiments may be made. It is understoodthat the specific methods, materials, conditions, process parameters,apparatus and the like do not necessarily limit the scope of theinvention.

Device Fabrication

Prior to device fabrication, indium tin oxide (ITO) on glass waspatterned as 2.2 mm wide stripes (sheet resistance ˜20Ω/□). Thesubstrates were cleaned by sonication in soap solution, rinsing withdeionized water, boiling in trichloroethylene, acetone and ethanol for3-4 min in each solvent. After the cleaning procedure, the substrateswere dried under an N₂ flow followed by UV ozone treatment for 10 min.

Organic layers of the OLEDs were sequentially deposited by thermalevaporation from resistively heated aluminum oxide crucibles onto thesubstrates at room temperature at a base pressure of ˜3-8×10⁻⁷ Torr, at2.5 Å/s. The rate of a single-component layer was controlled with oneInficon thickness monitor located close to the substrate. For thetwo-component emissive layer the rate of the dopant was controlled withan additional crystal monitor located close to the dopant evaporationsource. The additional monitor was not exposed to the major flow of thehost, which allowed increasing the precision of the dopantconcentration.

The devices were encapsulated and characterized after fabrication.Current-voltage measurements were made with a Keithley source meter(model 2400). Spectra and light intensity were measured using aPhotoResearch 705 model spectrophotometer and calibrated photodiode.

Example 1

An OLED was prepared with the structure ITO/Ir(Ph-ppy)₃(100 Å)/NPD(300Å)/CBP:Ir(Ph-ppy)₃ (75 Å, 12%)/CBP:Ir(pq)₂(acac) (25 Å,6%)/ADN:BFD47(300 Å, 6%)/Alq₃(300 Å)/LiF(5 Å)/Al(1000 Å). The devicestructure is depicted schematically in FIG. 3. FIG. 6 shows the plot ofthe external quantum efficiency versus current density for this device,which has a maximum external quantum efficiency of 4.7%. The plots ofthe normalized emission spectra at various current densities(brightness) are shown in FIG. 8. As can be seen in FIG. 8, the deviceshows a high color-stability of the light emission over a wide range ofcurrents or luminances.

Example 2

An OLED was prepared with the structure ITO/Ir(Ph-ppy)₃ (100 Å)/NPD(300Å)/CBP:Ir(Ph-ppy)₃(75 Å, 9%)/CBP:Ir(pq)₂(acac)(25 Å, 6%)/ADN:BFD47(200Å, 6%)/Alq₃(400 Å)/LiF(5 Å)/Al(1000 Å). The device structure is depictedschematically in FIG. 3. FIG. 6 shows the plot of the external quantumefficiency versus current density for this device, which has a maximumexternal quantum efficiency of 4.7%.

Example 3

An OLED was prepared with the structure ITO/Ir(Ph-ppy)₃(100 Å)/NPD(300Å)/CBP:Ir(Ph-ppy)₃(6%):Ir(pq)₂(acac)(1%)(100 Å)/ADN:BFD47(300 Å,6%)/Alq₃(300 Å)/LiF(5 Å)/Al(1000 Å). The device structure is depictedschematically in FIG. 4. FIG. 6 shows the plot of the external quantumefficiency versus current density for this device, which has a maximumexternal quantum efficiency of 5.4%.

Example 4

An OLED was prepared with the structure ITO/Ir(Ph-ppy)₃(100 Å)/NPD(300Å)/CBP:Ir(Ph-ppy)₃(6%):Ir(pq)₂(acac)(1%)(75 Å)/CBP(15 Å)/ADN:BFD47(300Å, 6%)/Alq₃(300 Å)/LiF(5 Å)/Al(1000 Å). The device structure is depictedschematically in FIG. 5. FIG. 6 shows the plot of the external quantumefficiency versus current density for this device, which has a maximumexternal quantum efficiency of 6.8%. The plots of the normalizedemission spectra at various current densities (brightness) are shown inFIG. 9. As can be seen in FIG. 9, the device shows a highcolor-stability of the light emission over a wide range of currents orluminances.

Example 5

An OLED was prepared with the structure ITO/Ir(Ph-ppy)₃(100 Å)/NPD(300Å)/CBP:Ir(Ph-ppy)₃(6%):Ir(pq)₂(acac)(1%)(50 Å)/CBP(10 Å)/ADN:BFD47(300Å, 6%)/Alq₃(300 Å)/LiF(5 Å)/Al(1000 Å). The device structure is depictedschematically in FIG. 5. FIG. 6 shows the plot of the external quantumefficiency versus current density for this device, which has a maximumexternal quantum efficiency of 4.7%.

Example 6

An OLED was prepared with the structure ITO/Ir(Ph-ppy)₃(100 Å)/NPD(300Å)/CBP:Ir(Ph-ppy)₃(6%):Ir(pq)₂(acac)(1%)(50 Å)/CBP(15 Å)/ADN:BFD47(300Å, 6%)/Alq₃(300 Å)/LiF(5 Å)/Al(1000 Å). FIG. 6 shows the plot of theexternal quantum efficiency versus current density for this device,which has a maximum external quantum efficiency of 5.3%.

Example 7

An OLED was prepared with the structure ITO/Ir(Ph-ppy)₃(100 Å)/NPD(300Å)/CBP:Ir(Ph-ppy)₃(12%):Ir(pq)₂(acac)(1%)(75 Å)/CBP(15 Å)/ADN:BFD47(300Å, 6%)/Alq₃(300 Å)/LiF(5 Å)/Al(1000 Å). The device structure is depictedschematically in FIG. 5. FIG. 6 shows the plot of the external quantumefficiency versus current density for this device, which has a maximumexternal quantum efficiency of 6.8%. The plots of the normalizedemission spectra at various current densities (brightness) are shown inFIG. 10. As can be seen in FIG. 10, the device shows a highcolor-stability of the light emission over a wide range of currents orluminances.

Example 8

An OLED was prepared with the structure ITO/Ir(Ph-ppy)₃(100 Å)/NPD(300Å)/CBP:Ir(Ph-ppy)₃(4%):Ir(pq)₂(acac)(1%)(75 Å)/CBP(15 Å)/ADN:BFD47(300Å, 6%)/Alq₃(300 Å)/LiF(5 Å)/Al(1000 Å). FIG. 6 shows the plot of theexternal quantum efficiency versus current density for this device,which has a maximum external quantum efficiency of 6.8%.

Example 9

An OLED was prepared with the structure ITO/Ir(Ph-ppy)₃(100 Å)/NPD(300Å)/CBP:Ir(Ph-ppy)₃(18%):Ir(pq)₂(acac)(1%)(75 Å)/CBP(15 Å)/ADN:BFD47(300Å, 6%)/Alq₃(300 Å)/LiF(5 Å)/Al(1000 Å). FIG. 6 shows the plot of theexternal quantum efficiency versus current density for this device,which has a maximum external quantum efficiency of 6.1%.

TABLE 1 measured @ 10 mA/cm² EQE % @ Example CIE x CIE y cd/m² V 10mA/cm² Max EQE % 1 0.33 0.32 900 6.7 4.7 4.7 2 0.35 0.34 910 7.1 4.7 4.73 0.36 0.32 1035 6.8 5.4 5.4 4 0.40 0.36 1360 7.2 6.7 6.8 5 0.34 0.32900 6.5 4.6 4.7 6 0.35 0.34 1000 6.6 5.0 5.3 7 0.38 0.35 1250 6.8 6.26.2 8 0.40 0.36 1230 7.1 6.1 6.8 9 0.39 0.35 1230 7 6.1 6.1

While the present invention is described with respect to particularexamples and preferred embodiments, it is understood that the presentinvention is not limited to these examples and embodiments. The presentinvention as claimed therefore includes variations from the particularexamples and preferred embodiments described herein, as will be apparentto one of skill in the art.

What is claimed is:
 1. An organic light emitting device comprising: acathode; a first emissive layer comprising a fluorescent blue emittingmaterial as a dopant in a host material; a spacer layer; a secondemissive layer comprising a phosphorescent emitting material as a dopantin a host material; and an anode; wherein, the spacer layer is betweenthe first emissive layer and the second emissive layer, the spacer layerinhibits quenching of the triplet excitons in the second emissive layer;an interface between the spacer layer and the first emissive layer formsa heterojunction; and the spacer layer comprises a material that has atriplet energy that is higher than the triplet energy of thephosphorescent emitting material; and wherein a combined emission of theemitting materials gives a white emission from the device.
 2. Theorganic light emitting device of claim 1, wherein the blue emittingmaterial is a blue emitting arylpyrene.
 3. The organic light emittingdevice of claim 1, wherein the blue emitting material is1,3,6,8-tetra(2-naphthyl)pyrene.
 4. The organic light emitting device ofclaim 1, wherein the blue emitting material is a blue emittingarylenevinylene compound.
 5. The organic light emitting device of claim1, wherein the device further comprises a separate non-emitting holetransporting layer.
 6. The organic light emitting device of claim 1,wherein the second emissive layer comprises a phosphorescent greenemitting material and a phosphorescent red emitting material.
 7. Theorganic light emitting device of claim 1, comprising in order: acathode; a first emissive layer comprising the fluorescent blue emittingmaterial; a spacer layer; a second emissive layer comprising aphosphorescent green emitting material; a third emissive layercomprising a phosphorescent red emitting material; and an anode.
 8. Theorganic light emitting device of claim 1, comprising in order: acathode; a first emissive layer comprising the fluorescent blue emittingmaterial and a fluorescent green emitting material; a spacer layer; asecond emissive layer comprising a phosphorescent red emitting material;and an anode.
 9. The organic light emitting device of claim 1, whereinthe device emits light having CIE coordinates of X=0.37±0.07, andY=0.37±0.07.
 10. The organic light emitting device of claim 1, whereinthe device has a maximum external quantum efficiency of at least about5.0%.
 11. The organic light emitting device of claim 1, wherein thedevice has a maximum external quantum efficiency of at least about 6.0%.12. The organic light emitting device of claim 1, wherein the hostmaterial of the first emissive layer and the host material of the secondemissive layer are different chemical compounds.
 13. The organic lightemitting device of claim 12, wherein the material of the spacer layer isdifferent than the host material of the first emissive layer.
 14. Theorganic light emitting device of claim 12, wherein the spacer layer iscomprised of the host material of the second emissive layer.
 15. Anorganic light emitting device comprising in order: a cathode; a firstemissive layer comprising a fluorescent emitting material; a spacerlayer; a second emissive layer comprising a phosphorescent emittingmaterial; and an anode; wherein the spacer layer inhibits quenching ofthe triplet excitons in the second emissive layer; an interface betweenthe spacer layer and the first emissive layer forms a heterojunction;and the spacer layer comprises a material that has a triplet energy thatis higher than the triplet energy of the phosphorescent emittingmaterial; and when a voltage is applied across the device light isemitted from the emitting materials.
 16. The organic light emittingdevice of claim 15, wherein the first emissive layer comprises a hostmaterial and the fluorescent emitting material as a dopant in the hostmaterial.
 17. The organic light emitting device of claim 16, wherein thefluorescent emitting material is a blue emitting material.
 18. Theorganic light emitting device of claim 17, wherein the fluorescentemitting material is a blue emitting arylpyrene.
 19. The organic lightemitting device of claim 18, wherein the blue emitting material is1,3,6,8-tetra(2-naphthyl)pyrene.
 20. The organic light emitting deviceof claim 15, wherein the blue emitting material is a blue emittingarylenevinylene compound.
 21. The organic light emitting device of claim15, wherein the second emissive layer comprises a phosphorescent greenemitting material and a phosphorescent red emitting material.
 22. Theorganic light emitting device of claim 15, comprising in order: acathode; a first emissive layer comprising the fluorescent blue emittingmaterial; a spacer layer; a second emissive layer comprising aphosphorescent green emitting material; a third emissive layercomprising a phosphorescent red emitting material; and an anode.
 23. Theorganic light emitting device of claim 15, comprising in order: acathode; a first emissive layer comprising the fluorescent blue emittingmaterial and a fluorescent green emitting material; a spacer layer; asecond emissive layer comprising a phosphorescent red emitting material;and an anode.
 24. The organic light emitting device of claim 15, whereina combined emission of the emitting materials gives a white emissionfrom the device.
 25. The organic light emitting device of claim 24,wherein the device emits light having CIE coordinates of X=0.37±0.07,and Y=0.37±0.07.
 26. The organic light emitting device of claim 15,wherein the device has a maximum external quantum efficiency of at leastabout 5.0%.
 27. The organic light emitting device of claim 15, whereinthe device has a maximum external quantum efficiency of at least about6.0%.
 28. The organic light emitting device of claim 16, wherein thehost material of the first emissive layer and the host material of thesecond emissive layer are different chemical compounds.
 29. The organiclight emitting device of claim 28, wherein the material of the spacerlayer is different than the host material of the first emissive layer.30. The organic light emitting device of claim 28, wherein the spacerlayer is comprised of the host material of the second emissive layer.